The borax bead test is based on the creation of a glassy bead that can variously change colour when put under a Bunsen flame, potentially revealing a whole series of compounds, depending on their chemical (electronic) features.
First of all a loop, eyelet, is made in the end of a platinum wire. Then it's loaded with borax or phosphorus salt and heating in the flame you get a clear, colorless glassy sphear, the so called "bead". The bead will be our reagent to identify specific cationic components (borax bead test) or cationic and anionic components (phosphorus salt bead test).
In the present article we're going to see how to carry out identification tests with this interesting method.
When we talk about "borax" we're referring to the compound with formula Na2B4O7 · 10H2O, IUPAC name sodium tetraborate decahydrate but also known as sodium tetraborate. As support to create the bead is commonly used a platinum wire that is firmly attached to the end of a glass stick. Before creating the pearl, it would be good practice to clean the wire moistening it with 6N HCl, to solubilize impurities such as any residual crust of previous uses. The hydrochloric acid is necessary to transform the various compounds eventually present in chlorides, usually volatile and thus eliminated in the flame. Residue of other substances could indeed alter the outcome of the test. Alternatively, you can clean the wire with borax itself, loading the loop with borax and heating in flame. The impurities will be adsorbed within the melted bead, which is then eliminated.
Now, we're going to consider from a chemical (and physical) viewpoint what happens when the bead is heated in flame.
The first thing we notice is the so-called "popcorn" effect, namely the swelling of the substance caused by the loss of crystallization water:
Na2B4O7 · 10H2O Na2B4O7 + 10H2O ↑
For further warming, tetraborate decomposes to give sodium metaborate and boric oxide.
Na2B4O7 2NaBO2 + B2O3
This is our pearl before the reaction with the unknown substance. It should appear clear and colorless. If it wasn't we must continue heating up to fusion and reloading with more borax. Alternatively, as mentioned earlier, we unload the bead onto the work surface, and we create a new one.
Recognition of the cationic component: bead test
We bring in our flame (bunsen-burner) the glass-like bead. We heat it up to incandescence and then we remove it from the flame. Then we let it cool slightly and then, with the bead (that's on the top of the wire, retained by the loop), we touch a small amount of unknown substance.
The real reagent of this type of assay is boric oxide, B2O3 .This reacts with the oxides of certain metals to give the corresponding metaborates. These impart to the bead particular color features that may be indicative about the cationic composition of our unknown substance.
In addition, the test may have different outcomes depending on which zone of the flame we select. In fact, in oxidizing flame we assist both to the formation of oxidized compounds (and then eventually with different colors) and to thermochromisms a physical phenomenon consisting in color change given by heating, whereas in the reducing flame we assist to the formation of reduced compounds (and then again eventually with different colors).
Let's see what are the reactions involved step by step:
- The salts or compounds present in the unknown substance for prolonged heating or action of the oxidizing flame itself, give the corresponding oxides (some example):
CuCO3 CuO + CO2↑
CuNO3 CuO + NO2 + 1/2 O2↑
Cu(OH)2 CuO + H2O↑
- These oxides react then with boric oxide to give corresponding metaborates.
Oxidizing flame: CuO + B2O3 → Cu(BO2)2 cupric metaborate green (hot), blue-green (cold)
Reducing flame (the reducing agent is elemental carbon powder): 4NaBO2 + 2Cu(BO2)2 + C CO2 + 2Na2B4O7 + 2Cu metallic copper → red-brown
According to the different color we can recognize different cations, and that's is the aim of this old but interesting assay.
When you are loading the borax bead with the unknown substance it is good to do so it is adsorbed by the bead just a minimal amount of unknown substance. An excess of the reagent may not react and therefore remain as an unreacted dark mass that does not allow to observe well the coloration assumed by the small bead.
This test is a dry assay. It allows to recognize the following substances: Cr, Mn, Ni, Co, Cu, Fe. The real test reagent is boric oxide (B2O3) which reacts with the oxides of these metals to give colored metaborates. It may be useful to bring the pearl both in oxidizing flame, to observe highest oxidation states coloration of metals and eventual thermochromisms, and in reducing flame, where the metaborates of the metals react with carbon powder (C) and are reduced to lower oxidation states if not, in some cases, the same metal in the elemental state (as copper in the example above).
Scheme of possible outcomes